The ants are one of the most successful groups of insects, and are of interest because they form advanced colonies. They belong to the order Hymenoptera, and are particularly close relatives of the vespid and scoliid wasps. The first known ants appeared sometime during the later Cretaceous period. They are physiologically distinguished mainly by having sharply elbowed antennae, and by having a bead-like pedicel formed from the first few abdominal segments, which in wasps are joined to the thorax. Also, most ants are wingless, although this varies between individuals in a colony rather than between species.

Ant colonies are eusocial, and are very much like those found in other such Hymenopterans, though the various groups of these probably developed sociality independently through convergent evolution. Eggs are laid by one or sometimes more queens, and most of these grow up to become wingless, sterile females called workers. Periodically swarms of new queens and males called alates are produced, usually winged, which leave to mate. The males die shortly thereafter, while the surviving queens either found new colonies or occasionally return to their old one.

Ants develop by complete metamorphosis, passing through larval and pupal stages before they become adults. The larval stage is particularly helpless - for instance it lacks legs entirely - because it does not need to care for itself. The difference between queens and workers, and between different castes of workers when they exist, is determined by feeding in the larval stage. Food is given to the larvae by a process called trophallaxis, where an ant regurgitates food held in a crop for communal storage. This is also how adults distribute food amongst themselves. Larvae and pupae need to be kept at fairly constant temperatures to ensure proper development, and so are often moved around various brood chambers within the colony.

A new worker spends the first few days of its adult life caring for the queen and young. After that it graduates to digging and other nest work, and then again to foraging and defense of the nest. These changes are fairly abrupt and define what are called temporal castes. In a few ants there are also physical castes - workers come in a spectrum of sizes, called minor, media, and major workers, the latter beginning foraging sooner. Often the larger ants will have disproportionately larger heads, and so stronger mandibles. In a few species the media workers have disappeared, so there is a sharp divide and clear physical difference between the minors and majors, sometimes called soldiers.

Ant communication is primarily through chemicals called pheromones, which, because most ants spend their time in direct contact with the ground, are more developed than in other Hymenopterans. So, for instance, when a forager finds food on his way home (found typically through remembered landmarks and the position of the sun), she will leave a trail along the ground, which in a short time other ants will follow. When they return home they will reinforce the trail, bringing other ants, until the food is exhausted, after which the trail is not reinforced and so slowly dissipates. A crushed ant will emit an alarm pheromone that in high concentration sends other ants nearby into an attack frenzy, and in lower concentration attracts them, while a few ants use what are called propaganda pheromones to confuse their enemies. And so forth.

Like other insects, ants smell with their antennae. These are fairly mobile, having as mentioned above a distinct elbow joint after an elongated first segment, and since they come in pairs provide information about direction as well as intensity. Pheromones are also exchanged as compounds mixed in with the food interchanged in trophallaxis, giving the ants information about one another's health and nutrition. Ants can also detect what task group (e.g. foraging or nest maintenance) each other belongs to. Of special note, the queen produces a special pheromone without which the workers will begin raising new queens.

Ants attack and defend themselves by biting, and in many species, stinging, in both cases sometimes injecting chemicals into the target. Of special note here is formic acid.

The most primitive ants are the army ants and driver ants, from South America and Africa respectively. These do not form permanent nests, but instead alternate between nomadic stages and stages where the workers form a temporary nest (bivouac) out of their own bodies. Most ants form stationary colonies, usually dug into the ground or some other hollow. Colonies reproduce either through nuptial flights as described above, or by fission, where a group of workers simply dig a new hole and raise new queens. Colony members are distinguished by smell, and other intruders are usually attacked, with notable exceptions.
other

Some ants will raid the colonies of other ants, taking the pupae with them, which once hatched act as workers in the raider's colonies despite not being genetically related to the queen. A few species, such as the warrior ants (Neivamyrex pilosus), have become utterly dependent on such slaves, to the point of being otherwise unable to feed themselves.

Some ants, called honeypot ants, have special workers called repletes who simply store food for the rest of the colony, generally becoming immobile with greatly enlarged abdomens. In Africa where they live they are considered a great delicacy.

Some ants are responsible for both good and bad. Solenopsis invicta is responsible for both acting as natural predators, especially benefficial to farmers, but can also act as a nuisance.

Weaver ants (Oecophylla) build nest in trees by attaching leaves together, first pulling them together with bridges of workers and then sewing them together by pressing silk-producing larvae against them in alternation.

Leafcutter ants (Atta) feed exclusively on a special fungus that lives only within their colonies. They continually collect leaves which they cut into tiny pieces for the fungus to grow on. These ants have several differently sized castes especially for cutting up the pieces they are supplied with into even smaller pieces.

Carpenter ants feed and damage wood by hollowing it out for nesting. Carpenter ants vary in size and color but are usually large (1/4-1/2 inch) and blackish.

Aphids, which secrete a sweet liquid called honeydew. Normally this is allowed to fall to the ground, but around ants it is kept for them to collect. The ants in turn keep predators away and will move the aphids around to better feeding locations.

Myrmecophilous or ant-loving caterpillars (blues, coppers, or hairstreaks), which are herded by the ants like cattle, led to feeding areas in daytime and brought inside the ants nest at night. The caterpillars have a gland which secretes honeydew if the ants massage them.

Some myrmecophagous (ant-eating) caterpillars secrete a pheromone which makes the ants think the larva is one of their own. The caterpillars will then be taken into the ants' nest where they can feed on the ant larvae.

Human beings have had a mixed relationship with ants through most of history. On the one hand, ants have often been used in fables and children's stories to represent industriousness and cooperative effort. They can also be important for clearing out insect pests and aerating the soil. On the other hand, they can become minor annoyances or major pests themselves when they invade homes, yards, gardens and fields. Some species, called killer ants, have a tendency to attack much larger animals during foraging or in defending their nests. Human attacks are rare, but the stings and bites can be quite painful and in large enough numbers can be disabling. These can be especially problematic when introduced into areas where they are not native.

Termites, sometimes called "white ants," are in fact not closely related to ants, though they have a somewhat similar social structure. They comprise the order Isoptera.